Desulfurization of molten ferrous metals

ABSTRACT

Method of desulfurizing molten iron through the use of calcium carbide having a particular sizing.

United States Patent [1 1 [111 3,929,464

Todd et al. Dec. 30, 1975 [54] DESULFURIZATION OF MOLTEN [58] Field of Search 75/53-56, 58

FERROUS METALS [75] Inventors: Lamar S. Todd, Tonawanda', Alan References Cited Fitzgibbon, Lewiston; Michael C. UNITED STATES PATENTS Carosella, Niagara Falls; Donald C. y Sanborn a" of NIY. 2,803,533 8/1957 Bienlosek 75/53 [73] Assignee: Union Carbide Corporation, New primary E p D Rosenberg York, Attorney, Agent, or FirmFrederick J. McCarthy, Jr.

[22] Filed: Aug. 31, 1973 21 Appl. No.: 393,505 [57] ABSTRACT Method of desulfurizing rnolten iron through the use 52 US. Cl 75/58; 75/53 calcum carbde havmg a part'cular [51] Int. Cl. C21C 7/00; C21C 7/02 5 Claims, 1 Drawing Figure l9 l5 I7 23 2| -'Z-''- 7 V :l

U.S. Patent Dec. 30, 1975 lllullll ll DESULFURIZATION O F MOLTEN FERROUS METALS The present invention relates to a method of reducing the sulfur impurity of molten ferrous metals. More particularly, the present invention is directed to the I desulfurization of molten iron through the injection therein of calcium carbide.

It is well known in the art to use additions of calcium carbide to molten iron in order to lower the sulfur content of the iron. U.S. Pat. No. 2,803,533, Bieniosek et a1 (1957 discloses a practice of injecting a fluidized stream of calcium carbide powder into molten iron to achieve desulfurization, and efficient calcium carbide per ton of iron. Such calcium carbide efficiency has been obtained with small quantities of iron and at relatively high final sulfur levels, but as later Bieniosek British Pat. No. 829,274 (1960) indicates, the injection of calcium carbide powder laden gas (powder size 1.168 mm to 0.074 mm) into molten iron requires a follow-up stream of carbide-free agitating gas to achieve reasonably efficient usage for lower final sulfur contents. U.S. Pat. No. 3,001,864, Muller et al (1961), also discloses the injection of a' fluidized stream of calcium carbide into molten iron specifying relatively high gas injection velocities to provide the vigorous agitation required for reacting molten iron with calcium carbide particles sized 100% minus 48 mesh (Tyler screen size.)

The foregoing calcium carbide injection practices leave something to be desired in view of requiring follow-up gas agitation and very high gas injection velocities.

It is therefore an object of the present invention to provide a one-step calcium carbide injection process for the desulfurization of molten iron requiring only moderate gas velocities and providing efficient calcium carbide usage and rapid desulfurization.

A method in accordance with the present invention comprises the injection of very finely divided calcium carbide in a gas stream at moderate velocities well below the surface of a bath of molten iron, whereby the sulfur content of the iron is efficiently reduced to 0.015% and lower without the requirement of followup agitation treatment. The present invention is suitable for the treatment of tonnage quantities of molten iron.

In practice of the present invention, finely divided calcium carbide is provided, substantially all of which is not larger than 200 microns, at least about 50 percent being not larger than about 50 microns and at least about percent being not larger than about 10 microns; and the aggregate of the finely divided calcium carbide having a Fischer sub-sieve number of of less than about 12.0. The preferred sizing for the finely divided calcium carbide is substantially all not larger than 200 microns, at least about 60 percent being not larger than about 50 microns and at least about percent being not larger than about 10 microns, and the aggregate of the finely divided calcium carbide having a Fischer sub-sieve number of less than 7.5. Calcium carbide material of this nature can be prepared by milling commercially available calcium carbide, e.g. fine grade calcium carbide, the particles of which are substantially all in the range of 0.6 millimeter to 0.074 millimeter.

Finely divided calcium carbide having the sizing described above in accordance with the present invention can be readily injected into molten iron using a device of the type schematically illustrated in FIG. 1.

With reference to FIG. 1, finely divided calcium carbide powder 1 is charged from container 2 via valve 3 into dispensing vessel 5 having at its lower portion a conventional fluidizing mechanism indicated at 7, which can be one of the various commercially available devices. Fluidizing gas, e.g. nitrogen, is introduced via valve 9 and transport injection gas, which can be nitrogen, or any other gas or moisture free gas such as dry air is introduced via valve 11 and carries the finely divided calcium carbide through flexible hose conduit 13 into lance 15, suitable made of 1 /4 inches l.D. standard extra-heavy steel pipe protected by refractory brick sleeve 17. As shown, lance 15 is positioned in vessel 19, shown as a conventional torpedo car, and immersed in sulfur-containing molten pig iron 21 covered by a slag layer 23. The vessel illustrated ordinarily contains from about 100 to 250 tons of molten metal at depths of from 4 to 9 feet.

In the practice of the present invention, the ratio by weight of calcium carbide to injection gas is maintained in the range of to 140 because a ratio less than about 75 results in excessively long injection times and a ratio greater than 140 results in undesirably poor desulfurization efficiency. Also, the calcium carbide containing gas is injected in the lower 75 percent of the depth of the molten iron. In actual practice the lower 10 percent has provided excellent results. The velocity of the gas entering the molten iron is preferably in the range of about 5 to 15 feet per second. With the aforedescribed combination of conditions, the sulfur content of molten iron, for metal amounts of to 200 tons, can be reduced to not more than 0.015% S, and lower, e.g. to 0.01% and lower, starting with initial sulfur contents ranging from 0.025% up to 0.075% S, at calcium carbide usages not exceeding 4 pounds per ton of iron per 0.01% S reduction, and with calcium carbide usage being suitably in the range of 1.5 to 4.0 pounds per ton of iron per 0.01% S reduction. This can be achieved without any after-treatment or supplemental agitation, and gas injection can be discontinued when the calculated required amount of calcium carbide in the abovementioned range has been introduced into the molten metal. The combination of conditions stated above is considered to provide the high efficiency of calcium carbide usage due to the higher reactivity of the very fine calcium carbide particles which travel and rise very slowly through molten metal because of their small size, and are not forced rapidly to the surface since the injection gas velocity is moderate, e.g., not more than 15 feet per second. The effective reaction time of the particles in molten metal is thus increased and the loss of fine particles at the metal surface, due to slag entrapment or otherwise, is decreased. In addition, excessive splashing and loss of metal thereby in the containing vessel is avoided due to the use of relatively low gas injection velocities.

The following example will further illustrate the present invention:

EXAMPLE Molten iron from a blast furnace in the amount of tons was provided in a torpedo car" vessel of the type shown in the drawing at atempera'tureof about 2530F. The analysis of the iron was 4.61% C, 1.03%

Si, 0.060 P, 0.047 S, 0.69% MnhA vertically oriented lance formed ofsteel pipe having an inner diameter of 1% inches was immersed in the molten iron' to a depth of about 2-3feet'fromthe bottom of the vessel. The height of the molten iron in the vessel was about 6-7 feet. Nitrogen gas was passed through the'injection lance at a rate of 13.6 cubic feet per minute and'a velocity of about 15 feet per second. Entrained in the nitrogen gas was finely divided calcium carbide sized in accordance with the present invention as indicated in the Table below: i

The ratio by weight of the finely divided calcium carbide, obtained by milling commercial finely divided (32 mesh) calcium carbide, to nitrogen, was about 127%. The injection of calcium carbide was continued for 8 minutes 45 seconds at which time the temperature of the molten iron was about 2510F. at the end of this treatment the sulfur content of the iron was 0.015%. The total amount of finely divided material injected into the iron was 9.4 lbs. per ton of iron (2.95 lbs. per ton of iron per 0.01% S); the rate of injection was'134.9 pounds per minute. The calcium carbide efficiency based on the CaC content of the material was 22.9%.

In a series of tests involving the treatment of more than 50 separate quantities of molten iron in the range of 100-200 tons, in accordance with the present invention, the average calcium carbide efficiency based on the CaC, content of the material used was about 21.5%

Additional tests using finely divided calcium carbide in accordance with the present invention have shown that with decreasing particle size and decreasing'Fischer sub-sieve number increased calcium carbide effi- 4 ciency is obtained. Tests using commercial finely divided calcium carbide designated 32 mesh (about 88% of particles in the range 590 microns to 74 microns) gave calcium carbide efficiencies which were less than about half .of that obtained with the practice of the present invention.

What is claimed is:

l. A method for lowering the sulfur content of molten iron from the range of about 0.025% to 0.075% to not more than 0.015% by the injection of calcium carbide into said molten iron said method not requiring agitation by gas injection subsequent to the injection of calcium carbideand requiringnot more than about 4 pounds of calcium carbide per ton of iron per 0.01% of sulfur removed in said range, said method comprising the steps of:

i. providing in a vessel molten iron containing sulfur impurity inan amount of at least 0.025%.

ii. providing finely vdivided particles of calcium carbide in a gas stream substantially all of said finely divided particles of calcium carbide being not larger than 200 microns, at least about 50 percent of said particles being not; larger than 50 microns and at least 20 percent being not larger than 10 microns and the aggregate of said particles of calcium carbide having a Fischer sub-sieve number of less than about 12.

iii. injecting said calcium carbide containing gas stream at a velocity of not more than about 15 feet per second into the lower 75% of the total depth of molten iron to thereby reduce the sulfur content of said molten iron and provide in said molten iron a sulfur content of not more than 0.015% upon termination of calcium carbide injection.

2. A method in accordance with claim 1 wherein said calcium carbide containing gas stream is injected into said molten iron by means of a vertically oriented tube substantially centrally located within said vessel.

3. A method in accordance with claim 2 wherein said calcium carbide containing gas stream is injected into the lower 10% of the total depth of molten iron.

4. A method in accordance with claim 1 wherein the ratio by weight of calcium carbide to gas in said gas stream is in therange of about 75 to 130.

5. A method in accordance with claim 1 wherein the injection of gas into said molten iron is discontinued when the total amount of calcium carbide injected into said molten iron reaches a value in the range of from about 1.5 to 4.0 pounds of calcium carbide per ton of iron per 0.01% of desired sulfur reduction.

I ax

" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 4 Issue Date December 30, 1975 Inventor) Lamar S. Todd et al.

\ It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

a r- I At Column 2, line 14 after "other" insert --inert-.

At: Column 3, line 34 delete "70" e I Signed and Sealed this iwenty-second Day Of June 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Alfesli'lg f? Commissioner OfPaIems and Trademark; I 

1. A METHOD FOR LOWERING THE SULFUR CONTENT OF MOLTEN IRON FROM THE RANGE OF ABOUT 0.025% TO 0.075% TO NOT MORE THAN 0.015% BY THE INJECTION OF CALCIUM CARBIDE INTO SAID MOLTEN IRON SAID METHOD NOT REQUIRING AGITATION BY GAS INJECTION SUBSEQUENT TO THE INJECTION OF CALCIUM CARBIDE AND REQUIRING NOT MORE THAN ABOUT 4 POUNDS OF CALCIUM CARBIDE PER TON OF IRON PER 0.01% OF SULFUR REMOVED IN SAID RANGE, SAID METHOD COMPRISING THE STEPS OF: I. PROVIDING IN A VESSEL MOLTEN IRON CONTAINING SULFUR IMPURITY IN AN AMOUNT OF AT LEAST 0.025%. II. PROVIDING FINELY DIVIDE PARTICLES OF CALCIUM CARBIDE IN A GAS STREAM SUBSTANTIALLY ALL OF SAID FINELY DIVIDED PARTICLES OF CALCIUM CARBIDE BEING NOT LARGER THAN 200 MICRONS, AT LEAST ABOUT 50 PERCENT OF SAID PARTICLES BEING NOT LARGER THAN 50 MICRONS AND AT LEAST 20 PERCENT BEING NOT LARGER THAN 10 MICRONS AND THE AGGREGATE OF SAID PARTICLES OF CALCIUM CARBIDE HAVING A FISCHER SUB-SIEVE NUMBER OF LESS THAN ABOUT
 12. III. INJECTING SAID CALCIUM CARBIDE CONTAINING GAS STREAM AT A VELOCITY OF NOT MORE THAN ABOUT 15 FEET PER SECOND INTO THE LOWER 75% OF THE TOTAL DEPTH OF MOLTEN IRON TO THEREBY REDUCE THE SULFUR CONTENT OF SAID MOLTEN IRON AND PROVIDE IN SAID MOLTEN IRON A SULFUR CONTENT OF NOT MORE THAN 0.015% UPON TERMINATION OF CALCIUM CARBIDE INJECTION.
 2. A method in accordance with claim 1 wherein said calcium carbide containing gas stream is injected into said molten iron by means of a vertically oriented tube substantially centrally located within said vessel.
 3. A method in accordance with claim 2 wherein said calcium carbide containing gas stream is injected into the lower 10% of the total depth of molten iron.
 4. A method in accordance with claim 1 wherein the ratio by weight of calcium carbide to gas in said gas stream is in the range of about 75 to
 130. 5. A method in accordance with claim 1 wherein the injection of gas into said molten iron is discontinued when the total amount of calcium carbide injected into said molten iron reaches a value in the range of from about 1.5 to 4.0 pounds of calcium carbide per ton of iron per 0.01% of desired sulfur reduction. 